Isner dropped the first set, but fought back to win 5-7, 7-5, 7-6 (7/4) in two hours and 27 minutes.

The Americans then survived two match points to claim the dead mixed doubles rubber in a match tiebreak, 6-2, 2-6, 11-9.

Williams, who will be aiming for her 19th Grand Slam singles title at the Australian Open later this month, had her hands full with 12th-ranked Pennetta in the early stages of her first singles match of the new year.

Pennetta raced through the first set in just 19 minutes, winning 6-0 as a listless Williams made a string of unforced errors.

After the fifth game, a weary Williams asked an amused chair umpire if she was allowed have an "espresso" coffee delivered to her courtside.

At the end of the set, Williams received the coffee and it had an immediate impact, as she broke the drought by winning the first game of the second set.

Although Pennetta made another two early breaks to lead 3-2 in the second set, Williams then kicked into gear, rattling off the last 10 games of the match in an early-season reminder of her brutal power.

The American won 0-6, 6-3, 6-0 and said she needed something to perk her up, having only arrived in Perth on Saturday.

"I am a coffee drinker and I didn't have mine this morning and I was just feeling it so, I had to get some coffee in me," she said.

"I told them to just give me a shot of espresso. I asked if it was legal, I was like 'is it legal to have espresso' because I never did it before but I needed some espresso and I needed to wake up, it's the jet lag."

Williams said she also struggled with the 10am start for the match, but was at least grateful the arena roof was closed as the temperature outside soared past 40 degrees celsius.